I would remove the grid from the 'solid rock' around the cave, but cover the water and land inside the cave.
I would also make the grid as light as I could to minimize the distraction from the rest of the map ... personally, I would create a white grid and set the opacity at 0% and gradually increase the opacity until the grid is just visible enough to be used ... typically around 5% to 20% opacity depending on the color of the map below it.

If possible, it might be nice if the white shimmer of the water covered both the light and dark areas of water so it looks more like ripples over deep water and less like a hole in the water. One way to do that might be to set up and blend your water color layers to create depth (without the white ripple water texture) and then overlay all of the water area with the water/ripple texture set to some percentage of transparency. This is a trial and error thing to find what looks good.

Good luck.